I have previously discussed the notion of pure allophony, contrasting it with the facts of alternations. What follows is a lightly edited section from my recent NAPhC 12 talk, which in part hinges on this notion.

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While Halle (1959) famously dispenses with the structuralist distinction between phonemics and morphophonemics, some later generativists reject pure allophony outright. Let the phonemic inventory of some grammar G be P and the set of surface phones generated by G from P be S. If some phoneme p ∈ P always corresponds—in some to be made precise—to some phone s ∈ S and if s ∉ P then s is a pure allophone of p. For example, if /s/ is a phoneme and [ʃ] is not, but all [ʃ]s correspond to /s/s, then [ʃ] is a pure allophone of [s]. According to some descriptions, this is the case for Korean, as [ʃ] is a (pure) allophone of /s/ when followed by [i].

One might argue that alternations are more entrenched facts than pure allophony, simply because it is always possible to construct a grammar free of pure allophony. For instance, if one wants to do away with pure allophony one can derive the Korean word [ʃI] ‘poem’ from /ʃi/ rather than from /si/. One early attempt to rule out pure allophony—and thus to motivate the choice of /ʃi/ over /si/ for the this problem—is the alternation condition (Kiparsky 1968). As Kenstowicz & Kisseberth (1979:215) state it, this condition holds that “the UR of a morpheme may not contain a phoneme /x/ that is always realized phonetically as identical to the realization of some other phoneme /y/.” [Note here that /x, y/ are to be interpreted as variables rather than as the voiceless velar fricative or the front high round vowel.–KBG] Another recent version of this idea—often attributed to Dell (1973) or Stampe (1973)—is the notion of lexicon optimization (Prince & Smolensky 1993:192).

A correspondent to this list wonders why, in a grammar G such that G(a) = G(b) for potential input elements /a, b/, a nonalternating observed element [a] is not (sometimes, always, freely) lexically /b/. The correct answer is surely “why bother?”—i.e. to set up /b/ for [a] when /a/ will do […] The basic idea reappears as “lexicon optimization” in recent discussions. (Alan Prince, electronic discussion; cited in Hale & Reiss 2008:246)

Should grammars with pure allophony be permitted? The question is not, as is sometimes supposed, a purely philosophical one (see Hale & Reiss 2008:16-22): both linguists and infants acquiring language require a satisfactory answer. In my opinion, the burden of proof lies with those who would deny pure allophony. They must explain how the language acquisition device (LAD) either directly induces grammars that satisfy the alternation condition, or optimizes all pure allophony out of them after the fact. “Why bother” could go either way: why posit either complication to the LAD when pure allophony will do? The linguist faces a similar problem to the infant. To wit, I began this project assuming Latin glide formation was purely allophonic, and only later uncovered—subtle and rare—evidence for vowel-glide alternations. Thus in this study, I make no apology for—and draw no further attention to—the fact that some data are purely allophonic. This important question will have to be settled by other means.

References

Dell, F. 1973. Les règles et les sons. Hermann.
Hale, M, and Reiss, R.. 2008. The Phonological Enterprise. Oxford University Press.
Halle, M. 1959. The Sound Pattern of Russian. Mouton.
Kenstowicz, M. and Kisseberth, C. 1979. Generative Phonology: Description and Theory. Academic Press.
Kiparsky. P. 1968. How Abstract is Phonology? Indiana University Linguistics Club.
Prince, A. and Smolensky, P. 1993. Optimality Theory: Constraint interaction in generative grammar. Technical Report TR-2, Rutgers University Center For Cognitive Science and Technical Report CU-CS-533-91, University of Colorado, Boulder Department of Computer Science.
Stampe, D. 1973. A Dissertation on Natural Phonology. Garland.